The lithium ion battery field has progressed since Sony commercialized the first lithium ion battery in the early 1990s. Lithium ion batteries are now widely used as power sources and energy storage devices in our daily life, especially in portable electronics. Despite the great success of lithium ion batteries up to now, new generation electronic products, such as ultra-thin and ultra-light devices, place ever higher demands on battery performance. In addition, the development of electric vehicles, hybrid electric vehicles and plug-in hybrid electric vehicles also benefits from further enhancement of battery performance. Innovations in battery technology may fulfill the ever-increasing demand for higher power/energy density, better rate capabilities and longer cycle life. Longer cycle life can allow a battery to withstand more charge/discharge cycles.
The energy density of the batteries can be expressed as ∫0Q V(q)dq/wt, where q is the state of electronic charge, V(q) is the voltage at the state of electronic charge q, Q is the total amount of electronic charge transported during battery cycling, and wt is the weight of electrode. Improving energy density of the batteries can thus be accomplished in three ways: increasing voltage, increasing capacity or reducing weight. Using a high voltage cathode is an effective way to increase the voltage of lithium ion batteries, since the working voltage of an anode has almost reached the working potential of lithium metal. A lithium ion battery typically works at voltages below 4.2V. A cathode which can work above 4.3V can be considered as high voltage cathode.
In a three electrode electrochemical cell (i.e., counter electrode, reference electrode, and working electrode), the counter electrode and the reference electrode may be different. The reference electrode establishes the electrical potential against which other potentials can be measured. The counter electrode is an electrode used in a reaction in which current is expected to flow. When lithium metal functions as both a counter electrode and a reference electrode, the potential of lithium metal is taken as 0. Currently graphite is used as anode and it works at 0.2V vs. Li metal.
By substituting part of Mn with Ni in the spinel LiMn2O4 structure, the working redox couple becomes Ni2+/Ni3+ and Ni3+/Ni4+ instead of Mn3+/Mn4+. The double redox couple from Ni has increased the working voltage of LiNi0.5Mn1.5O4 to 4.7V due to the increased binding energy of Ni 3d electrons, which was reported to be 0.5 eV higher than Mn 3d eg electrons. Among the high voltage cathode materials, LiNi0.5Mn1.5O4 is considered as one of the most promising candidates because this material's redox chemistries involve double redox couples, Ni2+/Ni3+ and Ni3+/Ni4+, where relatively high capacity can be obtained. One redox couple means one electron can be transported per ion, so double redox couples allow two electrons to be transported per Ni ion, leading to a higher capacity. Comparing with some other materials with only one redox couple, materials using double redox couples can have a higher capacity. With a high working voltage of 4.7 V and theoretical capacity of 146.7 mAh/g, LiNi0.5Mn1.5O4 can provide 20% and 30% higher energy density than traditional cathode materials LiCoO2 and LiFePO4, respectively.
Reducing the weight of the batteries is another way to enhance the energy density. Battery electrodes can be designed to replace or even eliminate the use of binders or current collectors in conventional battery electrode structure. Replacing binders or current collectors can involve using other materials which can function as binder or current collector in the battery such that traditional binders and current collectors do not need to be included in the battery. When traditional binders or current collects are no longer required, they can be eliminated from the battery. For example, when the structure of the electrode obviates the use of separate binders or current collectors.
These designs have stimulated a new trend of developing high energy density lithium ion batteries through light-weight electrodes. In conventional systems, the active materials in the electrodes are first mixed with a conductive additive and binders, and then coated onto metal current collectors. While those additives, binders and current collectors do not contribute to battery capacity, they can form a substantial portion of the total weight of the batteries. Even though carbon fiber or carbon cloth can replace current collectors to reduce the weight of the battery and enhance flexibility, the weight from conductive additive and binder still contributes to the weight of the battery. Carbon cloth is a cloth made from carbon fiber, and is not a traditional fabric.
Furthermore, growth of active material onto the carbon fiber or carbon cloth deviates from the commercial practice of active material synthesis and may be challenging to scale up.